Where it is not practicable to control an air contamination in the breathing zone of workmen by adequate exhaust ventilation, etc., and it is necessary for persons to be exposed to harmful amounts of dust, smoke, fumes or gases, personal respiratory equipment is frequently used to protect the worker. Such equipment is not, in general, suited for prolonged daily use because of the inherent features of discomfort and inconvenience to the wearer. For emergency or temporary situations, or until effective control of contamination can be developed and applied, personal respiratory protection is frequently used to protect the worker from harmful exposures.
One type of personal respiratory protection which is commonly used is a filter-type respirator. Filter-type respirators may be either chemical-filter respirators or mechanical-filter respirators. Chemical-filter respirators removed harmful constituents from air passing through them by chemical reactions, absorption and adsorption and include ordinary gas masks and chemical respirators. Mechanical-filter respirators remove harmful constituents from air passing through them by mechanical filtration and include dust, mist or fume respirators.
Unlike supplied-air or supplied-oxygen respirators, filter-type respirators may be used only in areas where there is sufficient ambient oxygen to sustain life. Oxygen is not supplied by filter-type respirators. The only function performed by filter-type respirators is to remove air contaminants.
Since filter-type respirators are primarily used for emergencies, respirators are carried by workers throughout their work day so they will be readily available if needed. However, due to ambient air contaminants in the atmosphere, the respirator's ability to remove air contaminants decreases from the day the respirator is first exposed to the atmosphere. Thus, it is possible that the effectiveness may end, even though the respirator was never used to protect a worker. Likewise, if an emergency occurs and the respirator is put into use to protect a worker, the respirator's utility in removing contaminants slowly decreases until it is no longer effective. At that time, the respirator offers no protection to the worker.
Filter-type respirators frequently are unsafe because a worker may assume the respirator is effective, when, in fact, it has lost its ability to remove air contaminants because it has been consumed in a prior emergency or because it has been depleted by removing ambient air contaminants. Visual inspection of a filter-type respirator will not indicate whether the respirator is effective or has been depleted. Attempts have been made to overcome this problem by adding color changing indicators to the filtering material.
For many gases, charcoal is used totally or partially as the absorbent. Since charcoal is solid black, a color change of the absorbent would not be detectable and would be useless. Thus, for the most common uses for filter-type respirators, coating the absorbent with a color changing indicator and enclosing it in a visible enclosure will provide insufficient warning to the user.
Canisters having end-of-service life indicators are shown in U.S. Pat. Nos. 1,537,519 to Yablick; 3,966,440 to Roberts; 4,155,358 to McAllister et al.; and 4,154,586 to Jones and Ayes.